The present invention relates to a rotary fluid cylinder which is used, for instance, to open and close the jaws of a chuck mounted at the extreme end of a spindle of a machine tool, and more particularly to a coupling mechanism therefor.
In a rotary fluid cylinder, a pair of rolling bearings are inserted between a rotary member and a fixed fluid feeding member which is coaxially mounted on the rotary member to feed the fluid to the rotary member even while the latter is being rotated. The fluid feeding member is liable to swing or vibrate when, for example, the rotary member inevitably makes a run-out (which is caused by a dimensional machining error within a tolerance of respective parts and the deflection thereof due to their weight) while it is rotated for rotation of the chuck. The fixed fluid feeding member is then liable to come into contact with the rotary member when the annular gap defined therebetween is small, which results in seizure of the related parts. Therefore, the above annular gap has been conventionally made sufficiently large to avoid the occurrence of the contact, which is against the recent request to make a machine tool compact. Further, when the annular gap is made larger, the amount of oil leaking through which is increased, which causes increase of heat generation during operation. In order to restrict the amount of oil leaking less than a predetermined amount, it becomes necessary, for instance, to increase the longitudinal length of the annular gap, which is also against the request of making a machine tool compact.
It has therefore been proposed to apply a pre-load to the outer ring of the rolling bearing, as has been disclosed in German Offenlegungsschrift DE 3306571 A1, to restrain the clattering of the rolling bearing so as to decrease a so-called radial-gap which is generally required, when employing the rolling bearing, for allowing the above clattering. The required radial-gap influences the required annular gap between the rotary member and the fluid feeding member. Accordingly, it is true that, with this constitution, the required radial-gap can be decreased and so it theoretically becomes possible to decrease the required annular gap to that extent.
In fact, however, it has not succeeded in attaining the desired results. That is, it was found from our experiments that, although the clatterings of the rolling bearings have actually been restrained, seizure of the related parts has occured when the annular gap is decreased correspondingly to a decrease of the required radial gap, and further, the amount of oil leaking has not been decreased as expected. In other words, we have had to have the sufficiently large annular gap as in conventional cylinders, and the clatterings of the rolling bearings were restrained and the required radial-gap was decreased.
A volumetric relationship of associated elements is represented by the following numerical formulas.
Numerical formula (1)
amount of oil leaking .varies.(annular gap).sup.3 /longitudinal length thereof PA1 heat generated by rotation .varies. PA1 [longitudinal length.multidot.(dia. of rotary member).sup.3 .multidot.(revolutional number.sup.2)]/annular gap
Numerical formula (2)
In the above formulas (1) and (2), the symbol ".varies." indicates --varies as--.
The above formula (1) says
The amount of oil leaking increases in proportion to the (annular gap).sup.3, but decreases in proportion to the longitudinal length of the annular gap.
The above formula (2) says
The heat generated by rotation increases in proportion to longitudinal length of the annular gap.times.(diameter of the rotary member).sup.3 (revolutional number).sup.2, but decreases in proportion to the annular gap.
As is apparent from the above numerical formulas, the longitudinal length of the annular gap must be increased in proportion to the third power of a ratio of change of the annular gap to keep an amount of leaked oil to the predetermined amount while the annular gap is increased. On the other hand, since heat generated during operation is increased in proportion to the longitudinal length of the annular gap and in inverse proportion to the annular gap itself, eventually, heat generation is greatly increased when the amount of leaked oil is kept to the predetermined amount.